A common problem in the operation of electronic devices is the generation of electromagnetic radiation within the electronic circuitry of the equipment. Such radiation results in “electromagnetic interference” or “EMI” and thus interferes with the operation of other electronic devices with a certain proximity.
A common solution to ameliorate the effects of EMI has been the development of shielding materials capable absorbing and/or reflecting EMI energy. These shielding materials are employed to localize EMI within its source, and to insulate other devices proximal to the EMI source. In fact, in the United States, EMI shielding materials must comply with the commercial Federal Communication Commission (FCC) EMC regulations to be sold as EMI shielding materials.
Moreover, an objective for all EMI shielding materials to be used in electronic devices is that they must not only comply with FCC requirements, but also should also meet Underwriter's Laboratories (UL) standards for flame retardancy. In this context, EMI shielding materials need be made flame retardant, i.e., achieving a rating of V-0 under UL Std. No. 94, “Tests for Flammability of Plastic Materials for Parts in Devices and Appliances” (1991), without compromising the shielding properties necessary for meeting EMI shielding requirements.
Several attempts have been made to provide EMI shielding materials (or gaskets) that exhibit flame retardancy. Typically, an EMI gasket is provided with a distinct flame retardant layer through the use of a highly viscous flame retardant material which is subsequently cured. For example, U.S. Pat. No. 4,857,668, which is directed to a conductive fabric-on-foam EMI gasket discloses a distinct flame retardant urethane layer being disposed on the interior side of the conductive fabric. Likewise, U.S. Pat. No. 6,248,393, which is also directed to a conductive fabric-on foam EMI gasket, discloses the formation of distinct flame retardant layer on the interior side of the conductive fabric by the delimited penetration of the fabric with a highly viscous flame retardant composition. The delimited penetration of the fabric allows the exterior of the gasket to remain substantially coating free and thus conductive. However, the coated portions of the fabric lose conductivity due to the flame retardant coating. Thus, while these gasket materials exhibit surface resistivity, any z-axis conductivity or bulk resistivity exhibited by these materials will be drastically diminished due to the flame retardant.
Thus, there is a need in the art for EMI shielding materials that are flame retardant and exhibit z-axis conductivity or bulk resistivity suitable for electromagnetic shielding applications. Accordingly, it is an object of the present invention provide flame retardant EMI shielding materials that exhibit z-axis conductivity or bulk resistivity and methods of producing these materials.